Our previous research showed that Ramadan fasting during pregnancy adversely affects the health of the offspring of Muslim women (Almond & Mazumder, 2009; Van Ewijk, 2009). In this paper, we use 1998-2007 data on the population of English school-children and study effects of prenatal exposure to fasting on the school performance of Muslims aged 7 and 11 (Key Stage 1 & 2 tests). Ramadan each year starts about 11 days earlier, which enables us to separate season-of-birth and relative age effects from effects of prenatal Ramadan exposure. We use exact date of birth to determine if students were in utero during Ramadan, but do not know whether the student’s mother had actually fasted. We therefore estimate intention to treat effects, which also helps circumvent any potential bias due to self-selection into who fasts during pregnancy. We find that both the math and verbal tests scores of Muslim students who were exposed to Ramadan fasting before birth are significantly lower than those of other Muslim students. Effects are strongest on children exposed in mid-gestation. The effect is robust to different specifications including controls for free school meal status. Our results suggest that Ramadan fasting during pregnancy can explain about 5 to 10% of the academic achievement gap between Muslims and White British students.